The choroid plexus cyst is one of many malformations of the central nervous system that can be detected in utero by ultrasonography. Choroid plexus cysts occur in 2.3% of fetuses. Because previous reports have shown an association between choroid plexus cysts and chromosomal anomalies, we analyzed 82 prenatally diagnosed cases, 65 of whom had chromosome analysis performed. Of the group, 6.2% had chromosomal anomalies of the trisomy 18 type. The remaining 17 cases were clinically normal at birth. These cases of trisomy 18 were also associated with multiple structural anomalies. Therefore, we suggest that after the diagnosis of choroid plexus cysts is made, a complete ultrasonographic survey of the fetal anatomy be performed. Fetal karyotype determination may be offered to patients, especially in the presence of structural anomalies.